1. Field of the Invention
This invention relates to a method of detecting semiconductor laser mode hopping. This invention also relates to a semiconductor laser beam source apparatus used for laser beam scanning or the like, wherein the method of detecting semiconductor laser mode hopping is employed.
2. Description of the Prior Art
In scanning of a recording medium with a light beam conducted for recording image information or the like on the recording medium or for reading out the information recorded on the recording medium, a laser beam emitted by a semiconductor laser may be used as the scanning light beam.
However, it is known that mode hopping arises with the semiconductor laser, depending on the operating conditions of the semiconductor laser. Specifically, in the semiconductor laser, different modes (i.e. different laser beam wavelengths) compete with each other under specific operating conditions (i.e. combinations of specific temperatures with specific light amounts), and mode hopping back and forth between one of the modes to the other is repeated under the specific operating conditions. FIG. 6 is a graph showing an example of the operating conditions under which mode hopping arises. In FIG. 6, mode hopping arises under the operating conditions corresponding to the dotted regions.
It is also known that, when mode hopping arises, the light amount of the laser beam fluctuates, and mode hopping noise arises. The magnitude of the fluctuation in the light amount of the laser beam caused by mode hopping is normally about 1%, and may increase to approximately 2% or 3% in the worst case. Therefore, in the case where laser beam scanning is conducted for processing of, for example, a continuous tone image wherein the maximum fluctuation in the light amount of the laser beam is generally required to be not larger than approximately 0.2% and mode hopping arises during the laser beam scanning, nonuniformity arises in the continuous tone image which is read out or recorded.
The inventors studied mode hopping of the semiconductor laser and found that, when mode hopping arises, both the light amount of the laser beam and the laser beam emission point (beam waist position) of the semiconductor laser change. Specifically, the laser beam emission point moves in the longitudinal direction, i.e. in the laser beam emitting direction, and in the transverse directions normal to the longitudinal direction.
Movement of the laser beam emission point has the adverse effect of causing jitter and pitch nonuniformity in light beam scanning.
As mentioned above, with the semiconductor laser, mode hopping may arise and cause undesirable changes in the condition of laser beam emission, such as changes in the light amount of the laser beam and changes in the position of the laser beam emission point.
Accordingly, in the case where light beam scanning is conducted by use of the semiconductor laser, it is desired to avoid mode hopping or to compensate for changes in the laser beam emitting condition such as changes in the light amount of the laser beam and changes in the position of the laser beam emission point. For this purpose, it is necessary to detect mode hopping arising with the semiconductor laser. By "detection of mode hopping" is meant the detection of the occurrence of mode hopping or the detection of changes in the laser beam emitting condition caused by mode hopping.